Charging devices for charging a variety of portable devices, such as a smartphone, a tablet terminal, and the like, which are equipped with a secondary battery (such as a lithium battery), via a USB (Universal Serial Bus) port has been wide spread.
The charging current that is supplied from the USB port is generally 1 A or less. Recently, power supply circuits and charging devices capable of supplying a charging current of 2.1 A to allow quick charging are being developed.
A variety of techniques related to such charging devices are often being proposed.
However, when the charging current is increased from 1 A to 2.1 A as described above, a voltage drop due to the cable resistance between a power supply circuit (power supply board) and a USB connector of a portable device to be charged is increased.
Due to the increased voltage drop, there is a problem that the requirement of a voltage reference (5 V±5%, i.e., 4.75 V−5.25 V) defined in the USB standard (USB power supply standard) is not satisfied.